Effective radiotherapy implies progressive depletion of the cancer cell population during treatment with concomitant repopulation of the normal tissues. In the belief that further improvement in the results in radiation therapy could result from a fuller understanding of underlying radiobiological mechanisms, an algorithm for simulating the changes in the cell populations of both normal tissues and tumors during fractionated irradiation has been developed and tested in the clinical context. The results obtained with a number or practical application of this program have been widely published (see bibliography). It is not normally possible however, to publish full listings and users' instructions for each of the programs employed. This proposal is designed to rectify this deficiency. The proposed monograph will describe methods for clinical evaluation of dose-time-volume relationships, radiation quality, fractionation, overall time, and field size as well as specific parameters for repair, recovery and repopulation; methods for computer simulation of changing cell population in irradiated tissues and tumors together with algorithms for derivation of parameters; and provide documentation relating to the programs used.